I used UT tweak tool to try the desktop mode.
After a little test with desktop mode, I changed back to mobile mode.
Then the OS crashed.
If each time of mode changed, should reboot the device?
I found double changes with tweak tool, only the first click works.
So I had to reinstall ubuntu touch. That's very trouble because of poor web speed.

My device: MEIZU mx4

@tera ,thanks for your information.
I had sovled by parted tool.
I had resized the cahe and system size.
The device works well.
Many thanks.